Cerebral vessels are injured by light and dye. This induces platelet aggregation. The effects of drugs on this phenomenon will be monitored in an effort to (1) test in vivo efficacy of agents known to inhibit aggregation in vitro; (2) shed light on fundamental mechanisms underlying aggregation; (3) comparisons of drug action in this model, will be made with action of the same drugs in other models of vascular trauma, such as pinching the vessel.